Date of Graduation
Master of Science in Geology (MS)
Walter L. Manger
T.A. (Mac) McGilvery
Second Committee Member
Ammonoid, Fayetteville, Imo, Paleontology, Pitkin, Stratigraphy
The late Mississippian lithostratigraphic succession in the southern Ozark region, northern Arkansas, include, in ascending order, the Meramecan Moorefield Shale, Batesville Sandstone (with Hindsville Member); Chesterian Fayetteville Shale and Pitkin Limestone (with Imo Member). This interval is now interpreted as a single transgressive-regressive, unconformity-bounded, eustatic, third order cycle (Stevens and Manger, 2018). The Moorefield lowstand wedge is bound by a basal unconformity and succeeded by the transgressive Batesville Sandstone with its Hindsville Member. This succession reflects accretionary deposition along strike of the rising strand line, likely as a barrier island system, as its equivalents are shale (basinward) and limestone (laterally along strike) (Stevens and Manger, 2018). A Maximum Flooding Interval is represented by the lower Fayetteville Shale. The upper Fayetteville Shale represents highstand, transitioning to regression represented by the Pitkin Limestone and its Imo Member, succeeded unconformably by Pennsylvanian (Morrowan) siliclastics of the Hale Formation (Stevens and Manger, 2018).
The carbonaceous black fissile shales of both the Fayetteville Shale and Pitkin Limestone, Imo Member, were originally thought to have been deposited in deep water basinal environments. However, recent study of ammonoid taphonomy and sequence stratigraphy indicate that the Imo is a regressional interval, and, was deposited in a much shallower environment than previously surmised (Stevens and Manger, 2018). Supporting evidence includes tempestite deposits of the Imo Member, Pitkin Limestone (Stevens and Manger, 2018), enrolled Paladin imoensis trilobite (Brezinski, 2008), and the taphonomy of fossil ammonoids, particularly Emstites fayettevillea (Gordon, 1965, 1964 imprint) and Anthracoceras discus (Frech, 1899) within the Fayetteville Shale and Imo Member of the Pitkin Limestone, respectively. The tempestite deposits contain deep shelf organisms, particularly trilobites, that are preserved enrolled in conglomerate lenses within the interval, suggesting a position above effective wave base allowing storms to scour the sea floor. This relationship is not seen in the lower Fayetteville shale. Furthermore, the Imo Member certainly lay above carbonate compensation depths as, indicated by calcite after aragonite pseudomorphs found within Anthracoceras discus chambers (Stevens and Manger, 2018). Fayetteville Shale ammonoids Emstites fayettevillea (Gordon, 1965) have only calcite and minor dolomite filling notably imploded septal chambers, likely due to water column overpressure.
Stevens, J. (2020). Fayetteville and Imo Shales: Punctuated Upper Mississippian Shallowing Upward Sequence, Southern Ozark Region, Northern Arkansas. Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/3815